Electrochemical, especially potentiometric sensors may include an ion-selective membrane in which a potential depending on the concentration of a certain type of ions develops during operation. The most popular example of such a sensor is the potentiometric combination electrode with a pH glass electrode. This includes a membrane made of a pH-selective glass which usually is lightly blow-attached to the end of a tube-like glass shaft, with the membrane closing the shaft at one end. Inside the shaft, there is an internal liquid or thickened, gel-type electrolyte that touches the membrane and usually includes a buffer system to set the electrolyte to a predetermined pH value. For measuring, the glass membrane is put in contact with a measuring liquid, which causes a hydrated layer to develop at the border to the measuring liquid that may accept or release hydronium ions (H+ or H3O+). At the borderline between the membrane glass and the measuring liquid, a dissociation occurs during which the alkali ions of the membrane glass are replaced by H+ ions from the measuring liquid, thus creating a plurality of hydroxyl groups in the hydrated layer. Depending on the pH value of the measuring liquid, H+ ions either diffuse from the hydrated layer, or into the hydrated layer. During measuring operation of the glass electrode, this occurs both on the surface of the membrane touching the measuring liquid and on the opposite surface of the membrane touching the internal electrolyte. Since the internal electrolyte shows a constant pH value, there is a potential difference via the membrane that is dependent on the pH value of the measuring medium. Details on the design and function of a glass electrode for pH measuring are known from H. Galster, “pH-Messung, Grundlagen, Methoden, Anwendungen, Geräte”, VCH Verlagsgesellschaft, Weinheim, 1990. The book also names suitable membrane glasses.
Depending on the type of measuring task the sensor may be used for, a wide range of membrane geometries are used. The spherical or spherical cap form is very common. However, cone-shaped membranes or flat membranes are desired for certain measuring tasks.
The manufacture of electrochemical sensors by their nature usually includes the generation of an ion-selective membrane and an assembly that includes a shaft connected to the membrane. For the glass electrodes as described above, a glass assembly is constructed by connecting the membrane made of analyte-sensitive glass, especially of a pH-sensitive glass to a glass shaft by slightly melting or blowing it. This glass assembly is then connected to other components of the sensor.
Such a glass assembly with an analyte-sensitive membrane, especially including a spherical or spherical cap form, may be manufactured manually using craftsman's glass blowing techniques, by placing a tubular, usually cylindrical body, in the following referred to as the dip pipe, into molten glass. When it is subsequently pulled out, it takes a roughly determined amount of liquid glass with it from the glass melt. The glass blower then feeds air into the dip pipe with his/her mouth, which allows the sphere or spherical cap to be blown from the absorbed amount of molten viscous glass. This process requires skill and experience as a glass blower.
From German Patent, DE 101 16 099 B4, a method and automated apparatus to manufacture a glass assembly is known. This procedure allows a dip pipe to be lowered in the direction of the surface of a glass melt, and the point of reaching the surface is determined by detecting an increase of the gas pressure inside the dip pipe when the dip pipe touches the surface of the glass melt. The dip pipe is then controlled to have its free end lowered to a certain, predetermined table dip depth in the glass melt, and then is pulled out again, taking a predetermined amount of molten glass with it in the process. A pump is then used to control the gas pressure inside the dip pipe after the dip pipe is pulled out of the glass melt according to a gas pressure curve saved in the controls that generates a spherical or spherical cap glass membrane from the molten membrane glass absorbed by the dip pipe.
The method known from DE 101 16 099 B4 which can automatically be executed by the apparatus described there, is, however, not suitable for the manufacture of flat membranes.